In many large duct systems where isolation of components therein is required to permit safe entry for their servicing, double louver dampers offer the advantage over dampers of the guillotine type in that they enable the gaseous flow through the system to be modulated.
Double louver dampers have, however, certain disadvantages among which is that the two series of blades are so spaced that they are independent of each other with the result that when the dampers are open, the blades of each louver cause a pressure dip to occur in the gas flow therein with a consequent increase in operating costs required to maintain the wanted flow rate. When such dampers are closed, the sealing of the blades is a requirement that, as far as I am aware, has not been satisfactorily attained.